If you already have milk, you will be fine just adding a baby. It should be the same as if your child goes through a growth spurt and nurses more. Within a few days, your milk will catch up. A newborn's stomach is only the size of a marble. By day 10 it is the size of a ping pong ball. Plenty of time for your milk to catch up to 2 babes! I didn't see that you had a 16 month old. If you notice him starting to wean off, you can start pumping every 3 hours before baby is born (might want to do that about 2 weeks before due date anyways). Good luck!